1. Field of the Invention
The present invention relates to a controller for controlling a motor which drives a spindle of a machine tool, the controller having a function for displaying motor load.
2. Description of the Related Art
In a machine tool having a spindle driven by a motor, a device for displaying motor load (or a load meter) is generally used, in order to judge or estimate whether processing under a certain processing condition such as a cutting condition can be carried out. For example, JP S58-120455 A discloses a device for displaying a ratio of a spindle load relative to a rated load, wherein the ratio is indicated in green when the ratio is not more than 100%, and the ratio is indicated in red when the ratio exceeds 100%.
JP H10-090011 A discloses a method for displaying a coordinate system including horizontal and vertical axes representing a rotation frequency and an output of a motor, respectively, wherein a marker for indicating present rotation frequency and output of the motor is displayed along with a curve indicating output characteristics of the motor.
There are two types of load meters as described above. One type is a load meter with reference to a maximum output (hereinafter, such a load meter is referred to as a maximum output-referenced load meter), which calculates and displays a ratio of a present motor output (or a motor output during processing) relative to a reference value, the reference value corresponding to the maximum output. In the maximum output-referenced load meter, “100%” means a load level, wherein the motor is stopped due to torque shortage when the motor is subject to load more than the load level. Therefore, it is easy for an operator to determine as to how much margin is left between the present output and the load level.
The other type is a load meter with reference to a continuous rating output (hereinafter, such a load meter is referred to as a continuous rating output-referenced load meter), which calculates and displays a ratio of a present motor output (or a motor output during processing) relative to a reference value, the reference value corresponding to the continuous rating output. In the continuous rating output-referenced load meter, “100%” means a load level, wherein the motor or an amplifier thereof is overheated when the motor is continuously operated while being subject to load more than the load level. Therefore, it is easy for an operator to determine as to how much margin is left between the present output and the load level.
In this regard, as exemplified in FIG. 8, when a base velocity (where the motor velocity transits from a constant torque region to a constant output region) of a maximum output characteristic (Vb) and a base velocity of a continuous rating output characteristic (Vb) coincide with each other, a ratio between values indicated by the above two types of load meters is constant at any motor velocity (or rotation frequency). Therefore, when the value of one of the load meters is given, the value of the other load meter can be determined by a simple conversion.
On the other hand, as exemplified in FIG. 9, when the base velocity of the maximum output characteristic (Vb1) and the base velocity of the continuous rating output characteristic (Vb2) are different, a ratio between values indicated by the above two types of load meters is varied depending on the motor velocity (or rotation frequency). Therefore, it is not easy to covert one load meter value to the other load meter value. In other words, in a motor as shown in FIG. 9, it is necessary to take into consideration the present motor velocity (or the motor velocity during processing) in order to convert one load meter value to the other load meter value, and thus it is difficult to simultaneously determine both of the load meter values.
In a motor having a characteristic as shown in FIG. 8, the continuous rating output can be determined by multiplying a maximum output value by a constant ratio. Therefore, an operator can easily know as to how much margin is left between the present motor output and the maximum output and between the present output and the continuous rating output. However, in a motor having a characteristic as shown in FIG. 9, even when the load meter value with reference to the maximum output is given, an operator cannot easily know as to whether the motor can be continuously operated at the present load (or how much margin is left between the motor output and the continuous rating output).
In the device of JP S58-120455 A, a load state of a motor can be color-coded according to load values. However, this document does not provide means to solve the above problem. Further, in order to realize a display method of JP H10-090011 A in a motor controller, it is necessary to use a graphic display for indicating a graph. Therefore, the method of JP H10-090011 A is disadvantageous in view of a graphic resource, a memory resource and a cost of the controller.